This invention relates to multimode optical waveguides having an optimum index gradient and to methods of making them.
The propagation of light waves in optical waveguides is governed by laws of physics similar to those that govern microwave propagation and therefore can be studied in terms of modes, each of which has its own propagation and electromagnetic field characteristics. Single mode waveguides are advantageous in that they are capable of propagating optical signals with very low dispersion, but due to the low numerical aperture and/or small core size of such fibers, it is difficult to efficiently inject optical signals into these waveguides. Multimode waveguides have larger core diameters and/or larger numerical apertures than signal mode waveguides. Multimode waveguides are therefore often the preferred medium for the transmission of optical signals since they can efficiently accept light from injection lasers and incoherent, broad spectral width sources such as light emitting diodes. However, thousands of modes propagate in multimode optical waveguides, each mode traveling at a slightly different group velocity. A short input pulse that is shared by many guided modes thus splits up into a sequence of pulses that arrive at the output end of the waveguide at different times. This type of pulse dispersion is the dominant cause of dispersion in typical multimode optical waveguides.
Optical waveguides initially consisted of a core of uniform refractive index surrounded by a layer of cladding material having a lower refractive index. In this type of prior art fiber, the time required for the various modes to travel a given longitudinal distance along the waveguide increases as the mode order increases. The delay distortion in such a fiber, defined as the difference in the times it takes the fastest mode and the slowest mode to traverse a given longitudinal length, is very large. Optical waveguides having cores with radially graded index profiles exhibit significantly reduced pulse dispersion resulting from group velocity differences among modes This dispersion reducing effect, which is discussed in the publication by D. Gloge et al, entitled "Multimode Theory of Graded-Core Fibers," published in the November 1973 issue of the Bell System Technical Journal, pp. 1563-1578, employs a radially graded, continuous index profile from a maximum value at the center of the core to a lower value at the core-cladding interface. The index distribution in this type of waveguide is given by the equation EQU n(r)=n.sub.c [1-2.DELTA.(r/a).sup..alpha. ].sup.1/2 for r.ltoreq.a
where n.sub.c is the refractive index at the center of the core, n.sub.0 is the refractive index of the fiber core at radius a, EQU .DELTA.=(n.sub.c.sup.2 -n.sub.0.sup.2)/2n.sub.c.sup.2
and a is the core radius.
It was initially thought that the parabolic profile wherein .alpha. is equal to 2 would provide an index gradient that would minimize dispersion caused by group velocity differences among the modes.
The aforementioned Gloge et al publication describes a further development wherein a reduction in pulse dispersion is said to be obtained if, instead of .alpha. being equal to 2, it is equal to 2-2.DELTA.. However, the theory concerning index gradients wherein .alpha. is equal to 2 or 2-2.DELTA. neglects effects introduced by differences between the dispersive properties of the core and cladding compositions.
U.S. Pat. No. 3,904,268--Keck and Olshansky describes a gradient index optical waveguide wherein the dispersive properties of the core and cladding are reduced. This patent teaches that the gradient index optical waveguide with minimal delay differences among the modes has an index profile given by EQU n.sup.2 (r)=n.sub.c.sup.2 [1-2.DELTA.(r/a).sup..alpha. ] r.ltoreq.a
where ##EQU4## n.sub.c is the refractive index at the center of the core, n.sub.0 is the refractive index of the core at r=a, .DELTA.=(n.sub.c.sup.2 -n.sub.0.sup.2)/2n.sub.c.sup.2 and N.sub.c =n.sub.c -.lambda..sub.0 dn.sub.c /d.lambda..sub.0.
The invention of U.S. Pat. No. 3,904,268 is valid regardless of the glass composition provided the core refractive index is well described by the foregoing over the spectral range over which the source operates. The technique of the patent is applicable for all binary or multicomponent glass-forming compounds.
In accordance with the present invention, an additional class of graded index optical waveguides is described which are superior to the optical waveguide of U.S. Pat. No. 3,904,268 in their information carrying capacity.
The wavelength dependence of pulse dispersion of optical waveguides is an important consideration. A waveguide which provides low pulse dispersion at several different wavelengths or over a range of wavelengths is superior to one which provides low dispersion at or near a single wavelength. In the invention of U.S. Pat. No. 3,904,268, in general, the waveguide has minimal dispersion at or near a single wavelength. By choosing the profile shape of the waveguide according to Keck-Olshansky, minimal dispersion can be obtained at any chosen wavelength. However, as shown in FIG. (4) of this application, at other wavelengths, the dispersion is significantly greater.
The article "Profile Synthesis in Multicomponent Glass Optical Fibers" by Kaminow and Presby, Applied Optics 16 Jan. 1977 and U.S. Pat. No. 4,025,156 of Gloge Kaminow and Presby show that by proper choice of glass composition, an optical waveguide can be synthesized with dispersion minimized either over a range of wavelengths or at two or more distinct distinct wavelengths.
U.S. Pat. No. 4,033,667, Fleming is related to the teachings of Kaminow, Presby and Gloge in teaching how a particular composition can have a uniform index profile over a range of wavelengths.
As is clear from the examples cited in the Kaminow-Presby article, the Gloge Kaminow and Presby patent, and the Fleming patent, their inventions apply to only certain limited compositions. FIG. 1 in the Kaminow-Presby paper shows that the P.sub.2 O.sub.5 concentration at r=o must be 11.5 times greater than the GeO.sub.2 concentration at r=a in order to achieve reduced pulse dispersion over an extended range of wavelengths. Although favorable from the viewpoint of dispersion, this composition is undersirable from the viewpoint of viscosity, thermal expansion, chemical durability and numerical aperture.
The same restrictions on composition are imposed by the teachings of Gloge and Presby. In their example, they find that the concentration of GeO.sub.2 at r=o must be eight times less than the concentration of B.sub.2 O.sub.3 at r=a. This restriction on composition makes it impossible to design an optical fiber with other important properties such as high numerical aperture, good thermal expansion and viscosity matches across the diameter of the fiber.
The present invention avoids the severe restrictions on composition which is required to practice the Gloge-Presby patent. As will be shown, a preferred embodiment of this invention is a graded index optical waveguide, having low dispersion over a range of wavelengths or at two or more different wavelengths, and fabricated from a broad range of possible compositions.
As an example of the usefulness of the present invention, consider the fact that installing communication cables is very expensive. The cost of optical waveguides is quite small compared to this installation cost. The installed cables may have state of the art waveguides which have minimum pulse dispersion at the wavelength of sources which are presently being used, typically about 0.85 .mu.m. In the future, sources may be developed which are more efficient at other wavelengths. It would be very desirable to use waveguides in cables presently being installed which will be capable of operating for a range of wavelengths. In this manner, the cost of future installation of cables with waveguides capable of operating at a different wavelength could be avoided.